Recovery of kraft white liquor with sulfur addition provided by calcium sulfate

ABSTRACT

The present invention relates to a method of recovering kraft pulping liquor. During causticization of the green liquor, calcium sulfate is added to produce a white liquor with a higher than normal active alkali content. Further, NaOH can be added to the liquor to balance the added sulfur content introduced by the CaSO4. The present method also eliminates the need for the conventional Na2SO4 addition to the black liquor.

United States Patent [72] Inventor Donald LeeCaldwell 3,523,864 8/1970 Osterman et al 162/30 Jackson, Tex. OTHER REFERENCES [21] P Fryling, Glenn R., Combustion Engineering Revised Edi- [22] Flled June 18, 1969 on, 9 p 27 6 [45 1 Patfmted 1971 Kress et al., The Paper Industry and Paper World January Midland, Mich.

Primary Examiner-S. Leon Bashore Assistant Examiner-Thomas G. Scavone [S4] RECOVERY OF KRAFI WHITE LIQUOR WITH AttorneysGriswold & Burdick, Stephen S. Grace and SULFUR ADDITION PROVIDED BY CALCIUM William R. Norris SULFATE 6 Claims, 1 Drawing Fig.

ABSTRACT: The present invention relates to a method of [52] US. Cl 162/30, recovering kraft pulping liquor. During causticization of the 23/48 23/185- 162/90 green liquor, calcium sulfate is added to produce a white [51] Int. Cl D21c 11/04 liquor with a higher than normal active alkali content. [50] Field of Search 162/29, 30; Further NaOH can be added to the iq to balance the 23/48 185 added sulfur content introduced by the CaSO The present [56] Reerences Cited method also eliminates the need for the conventional Na SQ,

add1t1on to the black liquor. UNITED STATES PATENTS 3,514,371 5/1970 Katsuo 162/29 x B/acfi WOOO/ f *b/OCA Washer hiyuor y q separa/ar Evaporo/or /Wh/'/e quo r:/Va0H oo O eu pu/ o Concen/ra/eo A/aOH b/ack figuor Ca//c 2.1;)?

6-11 0 6 Recovery I ./C/0/'///1s 0 w/;//e //'qu0/' u/fl ace 7 /6 reen uor Sme c/ar/fl'er Caus/fc/jer 9 2 O/SSO/Ver CaC0 /Ca/0//) C00) Ca/c/ner .S/aAer RECOVERY OF KRAFT WHITE LIQUOR WITH SULFUR ADDITION PROVIDED BY CALCIUM SULFATE BACKGROUND OF THE INVENTION The kraft process of converting cellulose materials, e.g., wood chips, into pulp for paper manufacture involves the digestion of the chips with a cooking liquor containing sodium hydroxide (NaOH) and sodium sulfide (Na S), so-called kraft White liquor. The white liquor reacts with the wood constituents to form a cellulose pulp which is separated from the spent liquor, i.e., kraft black liquor."

Due to the economics involved it is essential to recover and recycle certain components of the cooking liquor of the kraft process. Conventionally the black liquor is concentrated and then burned in a recovery furnace to remove organic impurities. Na SO is normally added to the concentrated black liquor prior to burning. In the recovery furnace Na O reacts with C and S to form Na CO and Na S which accumulate at the bottom of the furnace as a molten slag or smelt. The smelt is dissolved in water to form a green liquor," which is further treated to replenish the NaOH values for recycle as white liquor. Conventionally the green liquor is causticized by adding calcium hydroxide, i.e., slaked lime, which reacts with the Na CO; in the green liquor to produce NaOH and insoluble CaCO The regenerated white liquor is separated from the CaCO a process step known as clarification," and fed to the digester. The CaCO is calcined, slaked and recycled to react with more green liquor.

During processing of the cooking liquor, i.e., pulp digestion and liquor recovery, Na O and S values are lost or used up. For example, some sulfur esgapes withthe evaporation gases and organic gases in the recovery furnace. As mentioned above, Na SO is added to the concentrated black liquor be-, fore entry into the furnace. Some of this additive goes up the stack before reaction, causing inefficiency. Further extra elemental sulfur must be added in many cases since sulfur losses usually exceed sodium oxide losses. Thus it would be desirable to find a method of recovering kraft liquor which does not require the addition of Na SO Moreover, in the conventional kraft process, the reaction of Ca(OH) with Na- CO in the causticization step is incomplete. Equilibrium is reached at about 8085 percent conversion, thus leaving the regenerated, clarified white liquor with a substantial amount of inactive, undesirable Na CO carried over from the green liquor.

A principal object of the present invention is to provide a method of regenerating kraft pulping liquor.

A further object of the present invention is to provide a method of recycling kraft liquor which eliminates the need for the Na SO, addition.

A still further object of the present invention is to provide a method to improve the conversion of Na CO; during causticization of the kraft green liquor.

The Invention The above and other objects and advantages are found in the method of the present invention which comprises reacting the green liquor with a calcium sulfate-containing material, in addition to Ca(OH) It has been found that the addition of calcium sulfate to the causticization step unexpectedly increases the Ca(OH) -Na CO reaction efficiency. Furthermore, calcium sulfate itself reacts with Na CO to convert it to Na SO,a compound which is converted to active Na s in the recovery furnace on recycle.

The process is further improved by adding Na i) values in order to maintain a relatively constant Na 0:S ratio in the white liquor entering the digestor. Preferably, this is accomplished by adding free NaOH to the clarified white liquor.

The term calcium sulfate," as used herein, refers to anhydrite (CaSO any hydrated forms thereof such as hemihydrate (CaSO,. W4 0) and gypsum (CaSO,. 2H 0) and any materials containing calcium sulfate as a major component thereof.

The method of the present invention eliminates the need for the Na SO.,-S addition to the concentratediblack liquor. Calcium sulfate is added to the green liquor in the causticizer in an amount sufficient to make up the sulfur losses during processing. NaOH is added to the clarified white liquor in an amount sufficient to replace the Nil- 0 losses during processing. Since these losses are quite often not equal, the amounts of NaOH and Ca SO, can be varied independently giving the method a high degree of flexibility. Such flexibility is not possible where Na SO, is the major makeup material for both Na 0 and S losses.

The present invention is further understood with reference with the Figure, a schematic flow sheet of a particular embodiment of the present method.

Wood or other cellulose materials and kraft cooking liquor containing NaOH and Na S are reacted at elevated temperatures in a digester to produce pulp and black liquor. The pulp is washed and separated and the black liquor concentrated by evaporation. The concentrated black liquor is fed into a recovery furnace and the smelt product removed and dissolved in water to form green liquor. The green liquor is reacted in a causticizer with Ca(OH and calcium sulfate. The calcium sulfate is added in an amount sufficient to replace the sulfur values lost during processing (pulping and liquor recovery), i.e., the difference between the sulfur content of the white liquor and of the green liquor. A typical sulfur make up value is 7.1 percent of the total alkalinity in the green liquor, i.e., NaOH, Na CO and Na S constituents expressed as moles Na 0. The Ca(OH );,is added in at least the theoretical equivalent amount for complete conversion of the Na CO remaining unreacted with the Ca SO The causticization reaction produces NaOH, Na SO and an insoluble precipitate,

CaCO The CaCO is separated from the white liquor in the clarifier and can be recycled after calcination to react with more green liquor. The clarified white liquor is admixed with NaOH in an amount sufficient to make up the Na O losses to the liquor during processing to produce the kraft wh1te liquor. Surprisingly, the process of the present invenactive alkali, i.e., NaOH plus Na S.

The following examples serve to further illustrate the present invention. Unless otherwise expressed the numbers given are moles Na 0 equivalent and molar percent.

Examples 1-2 A 100 ml. sample of green liquor, i.e., dissolved solids from kraft pulpmill recovery furnace wherein the black liquor and added sodium sulfate were burned, was placed in a reaction vessel together with reagent grade CaSO,,. 2H 0 in an amount equal to 7.5 percent of the total alkalinity of the green liquor sample and reagent grade Ca(OH) in a 5 percent excess of the amount sufficient to react with that portion of the Na CO -,not required for reaction with the gypsum. The reaction mixture was stirred slowly at 90 C. for 2 hours. Mixture volume was maintained to plus or minus 5 percent by adding water. The reaction mixture was filtered and analyzed.

For comparison purposes, the same procedure was carried out on a second I00 ml. sample of green liquor except that the CaSO,was omitted. Sufficient Ca(OH)- was added to provide an amount of Ca values equivalent to the Ca values in the sample above. Thus this comparative example follows the conventional process.

Table I presents the results. Analysis of the starting green liquor shows the major constituent to be Na CO In the comparative example A, a portion (76.9 percent) of the Na CO is converted to NaOH producing a clarified white liquor with 84 percent active alkali. However, by adding calcium sulfate as prescribed in the present invention, the total amount of Na CO converted (90.8 percent) is significantly higher, and surprisingly the amount converted by the Ca(OH) (89.6 percent) is substantially increased. The difference is, of course, the amount of Na CO converted to the Na SO, by the CaSO.,.

TAB LE I Composition-Molar Equiv. lcrccnt conversion of" (as N820) We i-ifii Percent a e a A. Nit- C; by active Example NuzCUa NaOll NazS NagCOJ Ca(OH)z alkali Green liquor 0. 1204 0. 0130 0. 0463 32 (oinparativc-A 0. 0290 0. 1140 0. 0433 76. 0 76. 0 84 (ornparative-l. 0. 0110 0.1187 0. 0420 00. 8 80. 0 03 Comparative-2 0. 0248 0. 1164 0. 0438 80. 8 78. 4 87 n Assuming 100% reaction of CaSOr-illizt) with NazCOa.

. NaOH moles plus NazS moles I h 1 0 1 Hunt cuvo Na0l1 moles plus NarCOa moles plus NazS moles About 0.0120 moles NaOll (expressed as N310) added.

TABLE ll Composition-lilolar Equiv. Percent conversion of as 120 Percent NagCO; by active Example NtlzCOi NaOll NazS N02C03 Ca(011)z alkali (h'cvn liquor 0. 1320 0. 0170 0.0373 3 (onipnrnlivc-ll 0. 0238 (l. 1200 ll. 0342 82. 1 82. 1 87 (mnparnt vc-Zi 0. (H54 0.1100 0. 0331 88. 5 87. 0 01 lmnpzn'zitiVc-l 0. 0127 0. 1135 0. 0331 00. 6 80. 2 J2 The method of the present invention produced a clarified white liquor having 93 percent active alkali, over a 10 percent increase.

The example clearly shows the beneficial effect of the calcium sulfate addition to the causticizing step. To control the Na,0:S ratio in the white liquor and to replace any Na 0 losses, NaOH is added to the clarified white liquor or directly to the causticizer as previously described. Reagent grade NaOH equivalent to 6.3 percent of the total green liquor alkalinity (about 0.0120 moles) was calculated for the green liquor of table 1. This amount of NaOH was added directly to the causticizer in example 2. While the amount of Na CO converted is increased and the white liquor produced has increased active alkali, NaOH is preferably added after causticization and the white liquor is clarified, i.e., adding NaOH to the clarified liquor of example l.

Although these examples added CaSO and NaOH in amounts sufficient to replace the sulfur and Na 0 losses respectively, it is at once apparent that lesser amounts can be added and still obtain a white liquor with increased active alkali and increased conversion ofNa CO Examples 3-4 Samples of a different green liquor were causticized in a manner similar to example 1. To one sample was added only Ca(OH) in a 5 percent excess of the amount sufficient for theoretically complete conversion of Na CO comparative example B. To a second sample, anhydrite (CaSO in an amount equal to 7.9 percent of the total alkalinity of the green liquor was added in addition to the Ca(OH)- example 3. Still a third sample was treated in the same fashion except that gypsum (CaSO,. 2H 0) was used instead of anhydriteexample 4. After 2 hours of causticization the samples were filtered and the filtrates analyzed. The results are shown in table 11.

Once again, additions of calcium sulfate-containing materials to the causticization stage produced increased conversion of Na CO by the Ca(OH) thereby obtaining a clarified white liquor with substantially higher percent active alkali than the conventional process.

What is claimed is:

1. in the kraft white liquor recovery process which comprises the steps of burning a black liquor to form a solid residue, dissolving such residue in water to form a green liquor, causticizing the green liquor with Ca(OH) to convert the Na CO therein to NaOH to produce a white liquor and clarifying the white liquor; the improvement which comprises adding calcium sulfate, in addition to the Ca(OH) in the causticization step, thereby producing a white liquor with improved caustic values.

2. The improvement of claim I including the additional step of adding NaOH to the clarified white liquor.

3. The improvement of claim 1 wherein the calcium sulfate is gypsum.

4. The improvement of claim 2 wherein the calcium sulfate is added in an amount sufficient to replace the sulfur losses in the liquor during processing and the NaOH is added in an amount sufficient to replace the M1 0 losses in the liquor during processing.

5. A pulping process which comprises:

a. digesting cellulose materials with a cooking liquor containing NaOH and M 5, forming a pulp;

b. separating the pulp from the residual, spent, black liquor;

c. burning the black liquor to remove organic matter, leaving a solid residue;

d. dissolving the solid residue in water to form a green liquor, a major component of which is Na CO e. causticizing the green liquor by reaction with Ca(OH) and calcium sulfate, said CaSo being present in an amount sufficient to replace the sulfur losses during processing, the Ca(OH) being at least in an amount theoretically equivalent to react with the Na- CO remain ing after reaction with calcium sulfate, thereby forming a CaCO precipitate;

f. separating the precipitate from residual white liquor; and

g. adding to said white liquor sufficient NaOH to replace the sodium oxide losses during processing, thereby producing a white liquor substantially equivalent to the original white liquor used.

6. The process of claim 5 wherein the calcium sulfate is gypsum. 

2. The improvement of claim 1 including the additional step of adding NaOH to the clarified white liquor.
 3. The improvement of claim 1 wherein the calcium sulfate is gypsum.
 4. The improvement of claim 2 wherein the calcium sulfate is added in an amount sufficient to replace the sulfur losses in the liquor during processing and the NaOH is added in an amount sufficient to replace the Na20 losses in the liquor during processing.
 5. A pulping process which comprises: a. digesting cellulose materials with a cooking liquor containing NaOH and Na2S, forming a pulp; b. separating the pulp from the residual, spent, black liquor; c. burning the black liquor to remove organic matter, leaving a solid residue; d. dissolving the solid residue in water to form a green liquor, a major component of which is Na2CO3; e. causticizing the green liquor by reaction with Ca(OH)2 and calcium sulfate, said CaSo4 being present in an amount sufficient to replace the sulfur losses during processing, the Ca(OH)2 being at least in an amount theoretically equivalent to react with the Na2CO3 remaining after reaction with calcium sulfate, thereby forming a CaCO3 precipitate; f. separating the precipitate from residual white liquor; and g. adding to said white liquor sufficient NaOH to replace the sodium oxide losses during processing, thereby producing a white liquor substantially equivalent to the original white liquor used.
 6. The process of claim 5 wherein the calcium sulfate is gypsum. 